This invention deals with the problem of undesired reflections present in computer or television displays, especially in color displays. It is well known to reduce or suppress specular reflection by roughening the front surface of the display device, which, for example, could be the glass faceplate of a CRT, or a plastic overlay. However, when such a roughened surface is used in connection with a display screen which is made up of a regular pattern of fine dots or stripes, as is generally the case with direct view color displays, a disturbing phenomenon known as sparkle or random moire' arises: Interference between the spatial frequencies of the dot or stripe pattern and the similar spatial frequencies contained within the broad range of spatial frequencies that characterize a roughened surface, produces beats which appear to move when the observer moves, and which are quite disturbing.
Parent application Ser. No. 298,540 discloses a transparent overlay for color display devices, with an outer surface having sinusoidal ripples along its two major dimensions. Such a surface scatters reflected light and thus renders reflections much less disturbing. It produces no sparkle, no significant moire' and only a very small loss of resolution. This favorable performance is achieved by careful choice of the spatial frequency of the sinusoids, just high enough to be safely above the spatial frequencies of the phosphor dot pattern and of their lowest harmonics. Making the spatial frequency of the sinusoids no higher than what is necessary to avoid moire' preserves resolution by minimizing the diffraction effects.
Such a surface is most economically produced on a plastic overlay by pressing or rolling from a master, a process which is economical if the quantity produced is very large. In smaller quantities, such overlays are substantially more costly than overlays with random surfaces made by conventional spraying; these spray-generated surfaces, however, exhibit objectionable sparkle.
As used herein, "luminous" elements are elements or pixels from which light emanates or appears to emanate, including luminescent elements (cathodoluminescent or electro-luminescent, e.g.) and light-transmissive or light reflective elements (liquid crystal, e.g.).